Shaded pole motors are well known in the art and are characterized by a shading pole which is used to develop the starting torque for the motor. The general construction of a shaded pole motor includes a stator which is comprised of a stack of laminations so designed as to provide pairs of main poles, pairs of shading poles, wire slots for the magnet wire, slots through which to wind the magnet wire, slots for the shading bands and the shading bands themselves. Shading bands are generally comprised of one or more turns of copper or other electrically conductive material inserted into the shading band slots and shorted together to create a high current low voltage inductor. When the alternating current, and therefore the flux field, are increasing, a portion of the flux cuts the shading coil. This establishes a current in it which sets up a flux opposing the main field flux. Hence, at this time, lines pass only through the unshaded sections of the poles.
During the time the main flux is at its maximum value, the shading coil is not being cut. Then no opposing flux is established, and as a result the main field flux also passes through the coil.
During the time the main flux is decreasing, an EMF is induced in the shading coil, causing a current to flow, which sets up a flux in the same direction as the main field flux. Hence, a high flux passes through the shading coil.
The effect of a shading coil then is to cause a flux to sweep across the pole faces from the unshaded to the shaded section of the pole producing a rotating magnetic field which provides starting torque for the rotor.
As a portion of the shading band extends through the wire slot, it must be insulated from the electrical conductors forming each main motor pole. In the prior art, slot liners are used and are made from Mylar or paper. These slot liners are inserted into the slots and physically separate the electrical conductors from both the core and shading band. Although this prior art construction performs adequately, with the development of newer epoxies and other plastic materials, much attention has been directed to epoxy coating the slots including the shading band using various epoxy coating techniques in place of the slot liners previously used. However, there has been significant problems encountered in successfully epoxy coating the shading band and the gap between it and the underlying sidewall of the core. The reasons for this vary depending upon the epoxy coating technique utilized.
In the hot melt epoxy coating process, the stator is pre-heated to a temperature of around 400.degree. F. in an oven and then transferred to a coating station where the dry epoxy powder is sprayed onto those areas of the stator desired to be coated. As the powder settles onto the hot stator, it melts and flows into a continuous coating approximately 0.010 inches thick to create a smooth integral insulation which isolates the magnet wire extending through the slot from grounding against the steel stator. Unfortunately, the hot melt process tends to place a heavier coating on flat surfaces and a thinner coating on external corners and sharp edges. Since the magnet wire is wound over the sharp corners and edges forming the poles of the stator, the hot melt process deposits a minimum thickness at these very areas which increases the tendency for the magnet wire to wear the epoxy away and cause shorting to the core. As a partial remedy to this problem, several motor manufacturers have developed techniques for depositing excess amounts of powder on the corners and edges of the core to help minimize this tendency for the magnet wire to cut through the epoxy coating at those points.
Still another epoxy coating process is the electrostatic process in which the steel stator is not pre-heated at all but is instead placed in a fixture where it is electrically grounded to the frame of an epoxy coating machine. Typically, dry epoxy powder is floated in a fluidized bed and is ionized to create a 50,000 volt to 60,000 volt static charge. When the stator is positioned over this fluidized bed of ionized powder, the voltage potential therebetween creates an electrostatic charge which builds along the surfaces of the steel stator and causes the powder to stick to the stator by electrostatic deposition. The stator is then passed through an oven where the powder is melted and adheres to the stator in accordance with the relative thickness of powder distributed thereover. Since the electrostatic charge, and therefore the powder buildup, is naturally heavier on external corners and edge surfaces than on flat surfaces, this process naturally results in a heavier coating on the corners and edges than in the hot melt coating process. Consequently, the electrostatic epoxy coating process is generally considered much more desirable for use in insulating stator cores for shaded pole motors, as well as other electromagnetic devices.
Although electrostatic epoxy coating concentrates a heavier buildup on corners and edges, it is not as successful in applying an even coating to tight inside corners (about 90.degree. or less), or in bridging physical gaps between adjacent surfaces such as might be present between the edge of a shading band and the sidewall of the wire slot. As it is very common in the industry to use standard copper or other electrically conductive wire stock to form a shading band, the angles generally formed between the edges of the shading band and the wire slot sidewall are less than about 90.degree.. This is because standard copper wire stock typically comes in cross-sectional areas formed in the shape of a circle, square, rectangle, or flat with curved edges. The practice in the industry is to use copper wire stock as it comes from the drawing dies to form the shading band. In the prior art where paper and Mylar slot liners have been used, the shape of the shading band was of little consequence as the slot liner could be formed in such a way as to accommodate the particular copper wire shape used and still achieve a good insulation between the magnet wire and the shading band. However, with epoxy coating techniques, great difficulty has been encountered in achieving good deposition at the edge of the shading band adjacent the wire slot sidewall both because of the sharp angle therebetween and also the tendency of the shading band to "gap" away from the sidewall, especially in taller cores for larger motor sizes.
To solve these and other problems in the prior art, the inventor herein has succeeded in designing and developing a shading band with tapered edges and with a reverse bend formed in the shading band prior to its fixation to the core. This new design serves to virtually eliminate the gap between the shading band and the wire slot sidewall and also creates an effective angle between the edge of the shading band and the slot sidewall greater than about 90.degree. to thereby equalize the electrostatic charge buildup across the transition. The equalized charge also equalizes the electrodeposition of epoxy powder thereon to create a finished coat of epoxy having a more uniform thickness thereacross to thereby satisfactorily insulate the shading band and wire slot. As the inventive concept generally includes increasing the effective angle between the shading band and the wire slot sidewall, there are a multitude of shapes which the shading band can take and yet still achieve the purposes of the present invention. As is well known in the art, the electromagnetic effect created by the shading band is dependent upon the total cross-sectional area of the shading band which extends through the slot while the particular shape of the shading band has little effect. Consequently, various shapes are available for use including shading bands having a cross-sectional shape of a trapezoid, triangle, chord, hemisphere, or even odd compound angled shapes. Because of the physical limitations of the copper, and in order to preserve the structural integrity of the shading band, an upstanding edge is formed along the side edges where the shading band contacts the wire slot sidewall. This edge height should be controlled in order to control the effective angle formed between the shading band and the wire slot sidewall. This ensures that the electrostatic charge built up across the transition is increased which thereby results in an increased epoxy coating.
The inventor's solution to this difficult problem can be implemented by readily reforming those shading bands generally created in the prior art. Additional forming steps can be used to reshape the profile of the raw stock used to form a shading band at minimal expense such that both legs of the shading band are tapered and profiled into the same shape. Alternately, only one leg may be tapered and the other leg remains in a standardized shape for close fitting through a mounting slot in the stack of laminations and which is used to physically mount the shading band to the core. In this form, the present invention may be readily implemented without redesigning core slots in stator laminations or tools and dies used to form stator laminations.
While the principal advantages and features of the present invention have been described above, a greater understanding of the invention may be attained by referring to the drawings and description of the preferred embodiment which follow.